Electrical, Thermal, and Thermoelectric Transport Properties of Co-Doped <i>n</i>-type Cu_0.008Bi₂Te_2.6Se_0.4 Polycrystalline Alloys

Abstract

Bi&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;3&lt;/sub&gt;-based alloys have been extensively studied as thermoelectric materials near room temperature. In this study, the electrical, thermal, and thermoelectric transport properties of a series of Co-doped &lt;i&gt;n&lt;/i&gt;-type Cu&lt;sub&gt;0.008&lt;/sub&gt;Bi&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;2.6&lt;/sub&gt;Se&lt;sub&gt;0.4&lt;/sub&gt; polycrystalline alloys (Cu&lt;sub&gt;0.008&lt;/sub&gt;Bi&lt;sub&gt;2−x&lt;/sub&gt;Co&lt;sub&gt;x&lt;/sub&gt;Te&lt;sub&gt;2.6&lt;/sub&gt;Se&lt;sub&gt;0.4&lt;/sub&gt;, &lt;i&gt;x&lt;/i&gt; = 0, 0.03, 0.06, 0.09 and 0.12) are investigated. The electrical conductivity of the Cu&lt;sub&gt;0.008&lt;/sub&gt;Bi&lt;sub&gt;1.97&lt;/sub&gt;Co&lt;sub&gt;0.03&lt;/sub&gt;Te&lt;sub&gt;2.6&lt;/sub&gt;Se&lt;sub&gt;0.4&lt;/sub&gt; (&lt;i&gt;x&lt;/i&gt; = 0.03) sample was significantly enhanced, by 34%, to 1199 S/cm compared to 793 S/cm of the pristine Cu&lt;sub&gt;0.008&lt;/sub&gt;Bi&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;2.6&lt;/sub&gt;Se&lt;sub&gt;0.4&lt;/sub&gt; (&lt;i&gt;x&lt;/i&gt; = 0) sample at 300 K, and gradually decreased to 906 S/cm for &lt;i&gt;x&lt;/i&gt; = 0.12 upon further doping. Power factors of the Co-doped samples decreased compared to the 3.26 mW/mK&lt;sup&gt;2&lt;/sup&gt; of the pristine Cu&lt;sub&gt;0.008&lt;/sub&gt;Bi&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;2.6&lt;/sub&gt;Se&lt;sub&gt;0.4&lt;/sub&gt; sample at 300 K. Meanwhile, the power factor of the Cu&lt;sub&gt;0.008&lt;/sub&gt;Bi&lt;sub&gt;1.97&lt;/sub&gt;Co&lt;sub&gt;0.03&lt;/sub&gt;Te&lt;sub&gt;2.6&lt;/sub&gt;Se&lt;sub&gt;0.4&lt;/sub&gt; (&lt;i&gt;x&lt;/i&gt; = 0.03) sample became higher at 520 K. The lattice thermal conductivities of the Co-doped samples decreased due to additional point defect phonon scattering by the Co dopant. Consequently, the &lt;i&gt;zT&lt;/i&gt; for the Cu&lt;sub&gt;0.008&lt;/sub&gt;Bi&lt;sub&gt;1.97&lt;/sub&gt;Co&lt;sub&gt;0.03&lt;/sub&gt;Te&lt;sub&gt;2.6&lt;/sub&gt;Se&lt;sub&gt;0.4&lt;/sub&gt; alloy at 520 K was 0.83, which is approximately 15% larger than that of pristine Cu&lt;sub&gt;0.008&lt;/sub&gt;Bi&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;2.6&lt;/sub&gt;Se&lt;sub&gt;0.4&lt;/sub&gt;, while the &lt;i&gt;zT&lt;/i&gt; of the Cu doped samples at 300 K was smaller than that of the pristine Cu&lt;sub&gt;0.008&lt;/sub&gt;Bi&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;2.6&lt;/sub&gt;Se&lt;sub&gt;0.4&lt;/sub&gt; sample. Electrical transport properties of the Co-doped Cu&lt;sub&gt;0.008&lt;/sub&gt;Bi&lt;sub&gt;2−x&lt;/sub&gt;Co&lt;sub&gt;x&lt;/sub&gt;Te&lt;sub&gt;2.6&lt;/sub&gt;Se&lt;sub&gt;0.4&lt;/sub&gt; samples were analyzed by experimental phenomenological parameters, including the density-of-state, effective mass, weighted mobility, and quality factor.